The Study of Climate on Alien Worlds

It is a distracting, inconvenient coincidence that we are living in times of paradigm-shifting astronomical discoveries overshadowed by the deepest financial crisis since the Great Depression. Amid a battery of budget cuts, the astronomical community has discovered more planets outside of our Solar System—called extrasolar planets or simply exoplanets—in the past decade than in previous millennia. In the last couple of years alone, the Kepler Space Telescope has located more than 2,000 exoplanet candidates, including Earth-sized ones potentially capable of sustaining liquid water, demonstrating the ease at which nature seems to form them and hinting that we may be uncovering the tip of an iceberg. Discovering and characterizing distant, alien worlds is an endeavor no longer confined to the realm of science fiction.

In tandem with numerous surveys of the night sky performed from the ground, the Hubble, Kepler and Spitzer Space Telescopes observe the universe from outside of Earth’s atmosphere. These devices detect an exoplanet by recording the diminution of light as the body, residing in an edge-on orbit, passes in front of its host star. In the past few years, astronomers also have achieved the remarkable feat of measuring the diminution of light as the exoplanet passes behind its star, known as the secondary eclipse. In other words, astronomical techniques have advanced to the point where we can detect a star masking the light from its exoplanet, which is a demonstrably small effect—at most a few parts in a thousand in the infrared and much smaller in the optical range of wavelengths. During a secondary eclipse, the light from an exoplanetary system originates only from the star, and these data can be used to subtract out the starlight when the exoplanet is not eclipsed. All that remains is the light of the exoplanet and its atmosphere (if it exists). Such a technique has enabled astronomers to make the first detections of the light directly emitted by an exoplanet, which typically appears at its brightest in the infrared.

Measuring transits and eclipses at several different wavelengths allows one to construct a spectrum of the exoplanetary atmosphere, of which a spectral analysis yields its composition and elemental abundances. (A spectrum describes the range of colors of the photons emanating from the exoplanet, but it generally extends beyond what our eyes can see toward both shorter and longer wavelengths.) In some cases, astronomers were able to record the ebb and rise of the brightness of the exoplanet as it orbits its parent star, otherwise known as the phase curve. An inversion technique, developed by Nick Cowan of Northwestern University and Eric Agol of the University of Washington, allows one to convert the phase curve into a “brightness map,” which is the latitudinally averaged brightness of the exoplanet across longitude. Recent work by the same researchers has yielded two-dimensional information on the brightness of the exoplanet HD 189733b as a function of both latitude and longitude. In other words, we have started to do cartography on exoplanets!